1. Field of the Invention
The present invention relates to a method for forming a color conversion table, an apparatus for forming a color conversion table, a program fro forming a color conversion table and a printing apparatus.
2. Description of the Related Art
Image devices such as displays and printers usually produce colors using color image data which express three color components in terms of tone values. For example, they employ a color system such as the RGB color system which uses R (red), G (green), and B (blue) or the CMY color system which uses C (cyan), M (magenta), and Y (yellow). Since these color systems are usually device dependent, an LUT (lookup table) which defines the correspondence or correlation between color systems has been used in order to enable output of the same image with the same colors on different image devices. Such LUTs include ones which define the correlation between the color system of an input image device and that of an output image device, and ones which define the correlation between an output image device's color system and the absolute color space.
In a printing apparatus, usually an LUT must define the correlation by M3 coordinate points (where M denotes the number of grid points per color component). Since it is unrealistic to measure all points using a calorimeter, N3 coordinate points are picked up as sample data to determine M3 coordinate points (M>N). For example, in order to correlate Luv coordinate values with CMY coordinate values, N3 patches printed according to given CMY data are measured with a calorimeter to yield Luv coordinate data, and linear interpolation such as volume interpolation is performed based on the obtained correlation to work out CMY coordinate values which correspond to arbitrary M3 Luv coordinate values.
The above-mentioned conventional color conversion program has the following problem.
Since sample color patches are printed on the above-mentioned printing apparatus, CMY coordinate values of cubic lattice points can be selected in the CMY color system, but a lattice formed by coordinate values in a uniform color space such as Luv color system is generally distorted. In the above volume interpolation calculation, arbitrary coordinates in the cubic lattice can be converted into coordinates in the distorted lattice with high accuracy; however, accurate conversion of arbitrary coordinates in the distorted lattice into coordinates in the cubic lattice requires numerous lattice points and N3 coordinate points as mentioned above are not sufficient for this purpose. Therefore, an LUT thus formed is not accurate enough.
In an actual printing process, coordinate values are worked out by interpolation with coordinate values as defined by the above LUT and thus color conversion is not made accurately. Concretely, it is impossible to avoid a phenomenon that some area in the color space is less accurate than the other area, leading to a tone jump in the printed result. This method does not guarantee a certain level of accuracy for different types of printing apparatuses and the color conversion accuracy may be extremely low in some types of printing apparatuses